Randa U.S. Pat. No. 6,923,270, issued Aug. 2, 2005, the contents of which are incorporated by reference herein, describes a pneumatic ground piercing tool that includes a tail assembly including a tail nut and tail cap, the tail cap having a plurality of exhaust passages for exhausting spent compressed air. The discharge ports of the exhaust passages open into an annular space between a rearwardly opening recess of the tail cap and an air supply conduit such that the ports are shielded by a side wall of the tail cap from plugging when the ground piecing tool is operated in reverse mode. However, as shown in FIG. 9 of the patent, when seen from the rear view, the discharge ports are partly exposed. This permits dirt to enter the ports especially when the tool is operating in reverse mode. The present invention seeks to remedy this problem.